Glaucoma
Glaucoma, which is the leading cause of blindness in the United States, is a group of diseases characterized by progressive atrophy of the optic nerve head leading to visual field loss, and, ultimately, blindness. Glaucoma is generally associated with elevated intraocular pressure, which is an important risk factor for visual field loss because it causes further damage to optic nerve fibers.
There are several types of glaucoma, including open and closed angle glaucoma, all involve the abnormal increase in intraocular pressure, primarily by obstruction of the outflow of aqueous humor from the eye, or, less frequently, by over production of aqueous humor within the eye. The most prevalent type is primary open angle glaucoma in which the aqueous humor has free access to the irridocorneal angle, but aqueous humor drainage is impaired. In contrast, in closed angle glaucoma, the irridocorneal angle is closed by the peripheral iris. The angle block can usually be corrected by surgery. Less prevalent types of glaucoma include secondary glaucomas related to inflammation, trauma and hemorrhage. Glaucoma in its various forms is widely described in the literature: see, e.g., Leibrandt, ed. (1982) Professional Guide to Diseases, pp. 1203-1206 and Andreoli et al., eds. (1986) Cecil: Essentials of Medicine, pp. 690-691.
Therapeutic treatment of glaucoma is directed at reducing intraocular pressure. Because intraocular pressure is controlled by aqueous humor dynamics, an understanding of the production and removal of aqueous humor from the eyeball provides insights into the causes of increased ocular pressure associated with glaucoma. Aqueous humor is similar in electrolyte composition to plasma, but has a lower protein content. The aqueous humor keeps the eyeball inflated, supplies the nutritional needs of the vascular lens and cornea and washes away metabolites and toxic substances within the eye. The bulk of aqueous humor formation is the product of active cellular secretion by nonpigmented epithelial cells of the ciliary process from the active transport of solute, probably sodium, followed by the osmotic flow of water from the plasma. The nonpigmented epithelial cells of the ciliary process are connected at their apical cell membranes by tight junctions. These cells and the nonfenestrated iris vessels form the blood/aqueous barrier through which blood-borne large molecules, including proteins, do not pass.
Intraocular pressure is a function of the difference between the rate at which aqueous humor enters and leaves eye. Aqueous humor enters the posterior chamber by three means: 1) active secretion by nonpigmented epithelial cells of the ciliary process; 2) ultrafiltration of blood plasma; and 3) diffusion. Newly formed aqueous humor flows from the posterial chamber around the lens and through the pupil into the anterior chamber; aqueous humor leaves the eye by passive bulk flow at the irridocorneal angle and uveoscleral outflow. Any change in 1), 2) or 3) will disturb aqueous humor dynamics and likely alter intraocular pressure.